The present invention is particularly applicable to an inductive output tetrode (IOT) device such as a KLYSTRODE (Registered Trade Mark, Varian Associates Inc). The advantages of inductive output tetrode devices (hereinafter referred to as "IOT's") are well known but previously proposed designs have suffered from problems in that it has been necessary to provide a number of tubes, each of which may require to be used with a number of different cavities in order to provide the instantaneous bandwidth required (e.g. 8 MHz) over the entire television frequency range (e.g. 470-860 MHz). In klystrons, this requirement has been met by stagger tuning of the various cavities along the electron beam path to give outputs at different frequencies which add to give the required bandwidth. However, this is not possible with conventional IOT design.
It has been previously proposed to provide coupled output cavities for IOTs in which coupling is achieved between the two cavities by means of an adjustable aperture in a common wall. Variations in the coupling are limited to those that can be obtained by varying the size of the aperture.